h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Entry: The Electromagnetic Spectrum - A798717
Author: Bossel (back to business) - U132240

There are some numbers missing in the THz gap (to be fixed soon), but otherwise I think it's ready for comments. So please go ahead

Bossel

You've written a very detailed and complete entry, but you could consider concentrating on one part in detail, rather than try and cover the whole topic (I did this once, and it doesn't work ).

FYI, you also have posts at the end of your article.

Darn it Bossel - this means I'll actually have to dust off and finish my radio entry if you want to link to it

Hi Bossel,

I don't think the subject is too broad to cover in one entry.
I found a couple of little things.

"sun-milk" - we would say sun-screen or lotion.

Chunks and gizmo - I wouldn't use in a serious entry.

You say a little bit about the length of an antenna being related to the wavelength you want to detect. I assume it's not 1:1, but rather an exact multiple (or fraction)?

Awu.

Hi Boss,

Just been through this in full, and utterly ignorant of all things scientific, but a few language points for you:

"The shorter the path, the more cycles are completed in a second's time. The number of cycles per second is dubbed the frequency. "

- 'in one second' & 'called the frequency'; 'dubbed' is more like 'some people have decided to call it' in common usage.



"Charged somethings, being accelerated back and forth, emit radiation. "

- 'somethings' sounds very odd to me, here. Not sure what the right term would be, but not 'somethings'. Possibly, 'Anything that is charged'; but someone else may suggest a better way to say it.



"This radiation propagates, at the speed of light, into a direction that is perpendicular... "

- 'in a direction'; although I didn't know that anything could 'propogate' in a direction. I thought it was an intransitive verb. This may be a technical use of the word with which I'm not familiar, though.



"Any type of wave can be represented by an alternating series of 'crests' and 'troughs'."

- Some may say this is pedantic, but i'd say 'represented AS an alternating series of...'



"James C. Maxwell's equations could be put to practical realisation. "

- not sure what you mean by 'practical realisation' here. I *think* I know, but i'm not sure.



"This is because Earth's atmosphere exhibits dislikes and preferences when it comes to transmission of certain wavelengths."

- ok, i see what you mean, but it's not strictly accurate, is it? The Earth doesn't have 'feelings' on the subject, simply is not suitable for certain things.



"There are atmospheric windows where radition propagates with minor attenuation losses. "

- My scientific ignorance came particularly to the fore here. What on earth is that all about, .



"The colour pigments in human eyes are aligned in all polarisations without any preferences, otherwise we would have problems when looking at tilted LCD displays. Obviously, bees' eyes are built differently. "

- 2 points here:

- *Don't* we have problems seeing tilted LCD displays? I do. But praps i'm missing the point here.

- and, What do you mean about the bees' eyes? Nothing about that incongruous 2nd sentence seemed 'obvious' to me. Apart, perhaps, from the fact that, yes, it's obvious that bees' eyes *aren't* like human eyes. My ignorance again, no doubt, but confusing for 'this reader'.



"the range of energies (read: frequencies)"

- er... why suddenly start talking about 'energies', when i was just about getting my head round 'frequencies'? Shirley they're not 'the same thing'?



"As a consequence thereof,"

- I'd just scrap 'thereof'. You could replace it with 'of this' if you like, but i don't think it's needed here.



Okay, that seems like rather a lot, but i don't imagine it is all useful. I hope some of it is.

cyaround
spiff

Hello Bossel.

I know what you mean, but the energy thing needs some further explanation. There's a lot more energy available at 50Hz and 60Hz than at radio frequencies, isn't there.

OH

Bossel,

An entry I can understand!

Looks fine to me.

I have one comment to make: it doesn't really explain what the significance of 'electromagnetic' actually is. What *is* an electromagnetic wave? How is self propagating? Why doesn't it need a medium like every other kind of wave? Who worked all this out?

You and I know the answer to these questions, but the vast majority of readers won't.

An electromagnetic wave is just visible light, plus the bits off either side you can't see, like UV. The spectrum extends into Radio waves and TV, and in the other direction to X-Rays and Gamma Rays.

They're called waves because they act like waves: they can be diffracted, can overlap, and can do almost everything else a wave can.

To answer why it doesn't need a medium, it's probably easier to say why sound does need a medium. Sound, unlike EM waves, works by wiggling the air, just like waves in water wiggle bits of the sea. Your ears pick up the little wiggles in the air. EM waves don't wobble air. In fact, though they show properties of bing a wave, they also act like a particle, an this is the start of quantum physics.

EM waves are self-contained because they have a feedback loop going on with themselves. They wobble, which causes a chain of events which leads back to them wobbling. There is a reason for this, which I've forgotten, but it was a very good reason indeed, though a little complex and difficult to remember. Doh.

I hope that made it a little less clear.

Thanks for such a clear description, but I knew this already I was sort of making the point that it ought to go in the entry, even if only as a passing mention. If you're going to talk about electromagnetic waves, it would definitely help to describe what they are, and how they manage to span such a huge range of frequencies.

That's OK, Bossel's now got some stuff to copy-and-paste, if he/she so wishes (though I doubt Bossel will want to use the not exactly top quality explanation I've given).

Here, have some

I agree with everything Spaceman Spiff said. This saves me from transcribing most of my notes!

Also:

In your table you introduce a new abbreviation, eV, without explaining it. Is it described elsewhere in the Edited Guide?



When I see the prefix 'T' on a metric unit, I think Tera-. If the gap is that large, then it can't fit in your diagram as you have it listed, without a lot more explanation. Does the 'T' mean something else? Perhaps my puzzlement here is because I am having difficulty with 'dislikes and preferences' and don't know your allusion.



One way to detect x-rays is by emulsions of silver nitrite crystals, which is why x-rays have been an interesting tool for imaging ill people. There are fluorescent minerals that are useful for detecting other frequencies, too.



Are cosmic rays not electromagnetic? I'm sure I've heard of such things, but they may be actual particles like neutrinos, for all I know.

Mmf...thankew.....mmmffff

Uff! my peers!

And there was I hoping to slip an entry of fewer than 20kB source text into the Guide

Ok, here's what I've done so far:

Post 2, (concentrating on one part in detail rather than try and cover the whole topic) - Bob, this entry is meant to be a starting point for other entries which concentrate on one part. Therefore we (Hell and me) decided to keep descriptions as short as necessary.



Post 3, polishing off the 'radio' entry

yes Whisky, please (nudge, nudge)



Post 4 from Awü
sun-milk, Chunks and gizmo - done.

"You say a little bit about the length of an antenna being related to the wavelength you want to detect. I assume it's not 1:1, but rather an exact multiple (or fraction)?"
- Answering this question could escalate into a full-blown entry of its own . 'Basic' monopole antennas are integer fractions of the wavelength (ie: 1/4; 1/2; 3/4; 1/1) but then there are dishes, helices, patches and plates, dielectrics (lenses), and then there are *arrays* of dishes, helices, patches, etc. They can be more or less sensitive, they can be active or passive, they can be resonant or non-resonant, they can be omnidirectional, wide open or highly directional. All in all, it's simply impossible to give a general rule for antenna sizes other than they get bigger with the wavelength they are designed to receive or transmit.



Post 5, Spiff

"second's time", "dubbed", "somethings" - fixed I'd say 'objects' would cover it all.


"This radiation propagates, ... " - yes, propagation is also used in a technical sense. http://dict.leo.org/?p=lURE.&search=propagation has a whole page of applications


"Some may say this is pedantic, but i'd say 'represented AS an alternating series of...' - that's not being pedantic, it just appears to be proper English


"'James C. Maxwell's equations could be put to practical realisation.'
- not sure what you mean by 'practical realisation' here. I *think* I know, but i'm not sure.'"
-- I think the problem was the Denglish 'realisation' instead of 'application'!


"'This is because Earth's atmosphere exhibits dislikes and preferences when it comes to transmission of certain wavelengths."
- ok, i see what you mean, but it's not strictly accurate, is it? The Earth doesn't have 'feelings' on the subject, simply is not suitable for certain things.'
-- reworded, although sometimes I'd pretty much like to say that the Earth does have feelings...


"'There are atmospheric windows where radition propagates with minor attenuation losses."
- My scientific ignorance came particularly to the fore here. What on earth is that all about.'
-- hopefully, I've clarified this...


"The colour pigments in human eyes are aligned in all polarisations without any preferences, otherwise we would have problems when looking at tilted LCD displays. Obviously, bees' eyes are built differently. "

- 2 points here:
- *Don't* we have problems seeing tilted LCD displays? I do. But praps i'm missing the point here.
- and, What do you mean about the bees' eyes? Nothing about that incongruous 2nd sentence seemed 'obvious' to me. Apart, perhaps, from the fact that, yes, it's obvious that bees' eyes *aren't* like human eyes. My ignorance again, no doubt, but confusing for 'this reader'.
-- Hmmm... er, ... oh yes I see. It's nothing to do with *tilted* LCDs. LCDs emit polarised light, full stop. We can't tell the difference between polarised and non-polarised because our eyes don't differenciate, whereas bees' eyes can do the trick. The problem with a *tilted* LCD is a problem of the grooves and structures behind an LCD screen and the resulting decrease of intensity if the display is viewed from an angle.
I just tried that with a polarisation filter and it's true: turning the filter upright yields a bright screen, turning it to horizontal yields darkness. It is further true that a *dusty* LCD is a not-so-much polarised display


"the range of energies (read: frequencies)"
- er... why suddenly start talking about 'energies', when i was just about getting my head round 'frequencies'? Shirley they're not 'the same thing'?
-- rest assured, they *are*. Further up in the entry it was stated that energy is equivalent to frequency.


"As a consequence thereof,"
- I'd just scrap 'thereof'. You could replace it with 'of this' if you like, but i don't think it's needed here.
-- yup


"Okay, that seems like rather a lot, but i don't imagine it is all useful. I hope some of it is."
-- all of it *is*




Post 6
"I know what you mean, but the energy thing needs some further explanation. There's a lot more energy available at 50Hz and 60Hz than at radio frequencies, isn't there."

The point is that a single photon at 50Hz has 1/10 the energy of a photon at 500Hz, or 1/100 of a 5000Hz photon (and so forth). There's more energy available at 50/60Hz because power lines use maaaaany of those weak photons, ie: there's a high intensity within such a cable. Low frequency photons are easier to produce (you can easily shake a wire to produce a 2Hz photon, but you'd need to shake a wire until it glows red in order to produce an optical photon -- don't try this at home) and easier to guide along a wire, that's why power lines don't use, say, 200MHz.




Post 7

Thanks ZSF




Posted 21 Hours Ago by Felonious Monk; Man or Mondegreen?


Posts 8/9/10/11

What *is* an electromagnetic wave?

-- Fair points, everybody! I tried to keep the definition aside for an entry purely about electromagnetic waves, but obviously this doesn't work. I've now included two sentences and a link.




Post 12, Sea Change

Righto, Footnotes for 'eV' and 'Tera' added, 'diagnostics' (and link to x-ray entry) added to x-ray applications.

"Are cosmic rays not electromagnetic? I'm sure I've heard of such things, but they may be actual particles like neutrinos, for all I know."
-- Of course they are electromagnetic. It's just that the table entry got lost somewhere, sorry!



Bossel

Looking good!

I've only just read this. I find the constant switching between wave and particle confusing. Since you're talking about a spectrum, I think you would be better to deal with EM as a wave throughout the entry, with a short note at the end about the particle nature of light.

I'd prefer to see nm, μm, mm, m, km etc used instead of all those powers of 10, but I don't know whether they're commonly understood.

I think the term 'order of magnitude' needs some explanation.

Hello Bossel.

I've just read the entry through again, and found it a reasonable explanation of something rather technical, despite the following criticisms:-

I am no expert in this field (sorry for the pun), but the ideas about electrons in conductors are mostly um... misleading. Metals conduct heat rather well, if somewhat slowly, but not infrared or visible light. They also conduct electricity at high speed, but propagate radio waves rather badly (so electrical screening works). The EM wave does not require charged particles of any sort for its existence, and propagates well in "free space", i.e. in a vacuum. EM waves propagate through glass quite nicely, but charged particles do not, so you can unscrew a light bulb without being electricuted even if the power is on. The speed of oscillation of electrons in cables has little to do with conductor length - loudspeaker leads are usually a minute fraction of the various wavelengths involved, and aerial leads for UHF TVs work are usually a modestly high multiple of the various wavelengths involved, and these leads both work well.

"It took a long time for researchers to acknowledge ..." might be misinterpreted in h2g2, and perhaps "researchers" should be "the scientific community".

"Heat and brain waves" might be misinterpreted as "heat waves and brain waves", whereas "Brain waves and heat" would not be ambiguous in that way.

"heat and light are the only electromagnetic waves that humans can detect", so it must be the fact that 50Hz power lines are supplying low energy photons that people find shocking (and at 240V sometimes lethal).

In the text about the Terahertz gap, there is a typo: "windows where radition propagates" should be "windows where radiation propagates". More technically, in scientific notation due to plain text, if microwave frequencies are from 3E9 to 3E12 and infrared from 3E12 to 4.3E14, as the table states, where is the Terahertz gap between them?

"antenna" is an Americanism for the English "aerial".

"It is only the scale of length (from atoms and molecules to wires and antenna arrays) in which these processes take place that makes a difference." does not really explain all the various wavelengths of photons that emerge from ionised hydrogen and mercury (blues), or neon and sodium (reds).

As an aside because you're an editor, why are h2g2 messages from peer review not consistent: some have the form "Peer Review - Axxxx - Title", others just "Axxxx - Title". (I would prefer the first of these.) Also, this entry already has an editor, so how/when does that come about, why are there no clues in this thread, and since the original author is still active, why nothing from that author? (I have entries in peer review, none yet has an editor, and the first post has always been mine.)

OH

Old Hairy, I can answer some of your questions.

Threads in Peer Review are titled:

Axxxxxx - name of entry

When the entry is picked, the thread is moved out of peer review and attached to the end of the entry. The title of the thread is changed to include the words 'Peer Review' to it, to show that it is the thread that was the Peer Review thread.

If an entry has more than one contributor, the actual owner of the entry is listed as the Editor while the other contributors are listed as Authors. I assume that Bossel wrote this, but he is acknowledging a contribution from Hell. This unfortunately means that Hell is listed as the Author.

When the entry becomes picked, a new copy is made, which will have both Bossel and Hell as Authors and somebody else as Editor.

BOSSEL why the hell didn't you rewrite the whole thing.
Anyone searching for an antenna..just use a bit of wire and pump up the power. Creates havoc with the local taxi firms. Just love it!